It is well known in the art to use fluorescent dyes in dopant concentrations to monitor changes in certain specified characteristics of a given material. In particular, dyes have been used to probe molecular dynamics by chemically tagging them to a host molecule, to measure molecular orientation, to measure diffusion constants, to monitor polymerization, and to monitor the degree of cure of a curing thermoset material. In each of these applications, the fluorescent dye is chosen in accordance with the relationship between its spectral characteristics and the material property being examined.
For example, molecular rotor dyes such as 1-(4-dimethylaminophenyl)-6-phenyl-1,3,5 hexatriene (DMA-DPH): ##STR1## have been used to monitor the cure of thermoset materials, and to monitor polymerization. These dyes display fluorescence spectra whose intensity is dependent on the viscosity in the molecular neighborhood of the dye. Upon absorbing excitation energy, a molecular rotor can decay to its ground state via fluorescence radiation or by radiationless decay, i.e. energy transferred to molecular vibrations or rotations. For DMA-DPH, the amount of radiationless decay is regulated by rotations about the chemical bond at the end group as depicted by the arrow. This intramolecular rotational motion creates potential radiationless decay paths if its relaxation time, .tau..sub.r, is shorter than or approximately equal to the fluorescence decay time .tau..sub.f of the dye, usually tens of nanoseconds. For .tau..sub.r &gt;&gt;.tau..sub.f, maximum fluorescence radiation and minimum radiationless decay is observed.
Other types of dyes which may be used to analyze changes in various material properties are those which emit both monomer and excimer fluorescence. The radiated fluorescence energy is distributed between monomer and excimer modes of decay in accordance with the viscosity of the neighborhood. An example of this type of dye is bis(pyrene) propane (BPP) which contains two fluorescent pyrene molecules joined by a flexible propane linkage as shown below: ##STR2## When one pyrene molecule absorbs excitation energy at 340 nm, two paths of fluorescence decay are possible. The first is by monomer decay at 380 and 400 nm; that is, the pyrene molecule displays its characteristic fluorescence without interaction with the other pyrene. The second is an excimer decay in the range of 450 to 550 nm which occurs when the excited pyrene forms an excimer with its unexcited pyrene neighbor by rotating to a position of close molecular contact. The probability that the excited pyrene can move into the proper position to form an excimer before its own decay occurs is dependent on .tau..sub.r, the intramolecular rotational relaxation time of the dye, which is proportional to the ratio .eta./T where .eta. is a microscopic or molecular viscosity in the neighborhood of the dye and T is the temperature. Thus, for excimer producing dyes as well as for molecular rotors, rotational relaxation time plays a prominent role in the production of fluorescence radiation.
U.S. Pat. No. 5,037,763 to Petisce and U.S. Pat. No. 4,651,011 to Ors disclose methods of using fluorescent dye molecules incorporated into a curing material to measure the increase in viscosity which arises during a chemical change accompanying epoxy curing or polymerization. However, these methods require background corrections, absolute intensity measurements or uniform mixing of dye and resin. Moreover, these methods do not consider viscosity changes which occur during an accompanying phase transition in a chemically stable polymer, and they do not consider measurement of free volume cell size, heat of crystallization or crystallinity.